    Patent Document 1: JP-A H09-151342    Patent Document 2: JP-A 2006-124556    Patent Document 3: JP-A 2008-1796    Patent Document 4: JP-A 2005-238342    Patent Document 5: JP-A 2003-26972    Patent Document 6: JP-A 2006-104448    Patent Document 7: JP-A 2006-193652    Patent Document 8: JP-A 2006-342304    Patent Document 9: JP-A H11-35399    Patent Document 10: JP-A 2004-91560    Patent Document 11: JP-A 2004-49957    Patent Document 12: JP-A 2006-276271    Patent Document 13: JP-A 2006-335970    Non-Patent Document 1: “Nanotechnology Handbook,” first edition, edited by Nanotechnology Handbook Editorial Committee, Ohmsha Ltd., May, 2003, p. 13
The nanotechnology attracts a great deal of attention as a scientific technology raising a new industrial revolution. Conventional materials can exhibit new functions by mere microparticulation, so the nanoparticles become an important theme in the industrial world, and the advance of nanotechnology is naturally inseparable from microparticles, particularly nanoparticles.
Pigments are utilized in a very wide range of fields such as coating materials, printing inks, toner, inkjet inks and color filters, because of their vivid color tones and high colorability. Among these uses, inkjet pigments and color filter pigments are regarded as being important particularly in fields requiring practically highly functional materials.
Because a conventional substance can exhibit new functions by converting the substance into microparticles, the conversion of the substance into nanoparticles is an important theme in the whole industrial world, and the interest in the technology of conversion to nanoparticles is extremely increasing for advance of the nanotechnology (Non-Patent Document 1).
A variety of new functions attained by conversion to nanoparticles are known, and particularly pigment nanoparticles obtained by converting pigments into nanoparticles are known to attain coloration and color development equivalent to those of dyes.
As the color material of an inkjet ink, dyes have been mainly used from the viewpoint of storage stability for a long time, discharge stability from inkjet nozzles, excellent chroma and transparency. However, dyes are problematic in respect of light resistance and water resistance.
As compared with dye inks, pigment-dispersed inks using a pigment as their color material are known to significantly improve light resistance and water resistance and are useful as a material for producing inks for inkjet printers in a bubble jet (registered trademark) system, a thermal jet system and a piezo system and for producing writing utensils such as an aqueous ballpoint pen, a fountain pen, an aqueous marking pen and an aqueous marker, but uniform microparticulation of the pigment into fine nanoparticles capable of permeation into pore spaces in the surface of paper is difficult, so there is a problem that the pigment is inferior in adhesion to paper and in color rendering properties. Moreover, when the conventional pigment-based recording liquid is used in recording on transparent manuscripts represented by an OHP film, pigment particles are scattered to cause problems such as deterioration in transparency and dull coloration. The particle size of such pigment particles as the color material is known to exert a significant influence on performance, so there is a demand for a technology for stably producing monodisperse pigment nanoparticles (Patent Document 1). In production of color filters, the inkjet system is used in some cases.
With progress toward high resolution and high contrast in LCD monitors, organic EL monitors and digital cameras, it is desired that color filters used therein be thin-layer and have higher contrast and higher transmittance. Pigments are used in these color filters, and the particle size of the pigments used significantly influences on the thickness, contrast and transmittance of the resulting filter, so there has been a demand for a technology for stably producing monodisperse pigment nanoparticles (Patent Document 3).
The pigment particles used therein are obtained often by a step of pulverizing bulk pigment materials mechanically with a dispersing machine such as a ball mill or a bead mill, but such a pulverization step is problematic in that the average particle size of pigment particles obtained after the step is generally about 100 nm (0.1 μm) and the particle size distribution is relatively broad (about 80 nm to 180 nm) (Patent Document 2).
Moreover, microparticles produced by the pulverization method have active sites generated on their fracture surfaces as a result of pulverization, so that the pulverized microparticles are re-aggregated to form large particles and to increase the viscosity of the dispersion system as a whole, and therefore, there are many problems of the pulverization method itself.
There is a method, such as laser ablation shown in Patent Document 4, of radiating a femtosecond laser to solid particles, but because the laser ablation method uses a very strong laser beam, the possibility of decomposition at the molecular level is undeniable, and the amount of production at present is about 0.1 mg/h, which has not reached an industrial practical level.
As a production method other than the pulverization method, there is a growth method. Known is a method wherein a solution having an organic pigment dissolved therein is contacted gradually with an aqueous medium to separate the pigment (called co-precipitation method or re-precipitation method), which includes allowing a dispersant to be coexistent in either solution to prepare stable microparticles (Patent Document 5). In this method, particles of submicrons to several ten nanometers can be relatively easily produced, but are not suitable as the color material used in an inkjet ink or the like because the size of the particles can vary or amorphous particles other than spherical particles are easily formed due to scale-up.
A method of forming pigment nanoparticles, known as a microchemical process, is known wherein a micro-reactor or a micro-mixer is combined with a step of producing a pigment dispersion, such as a pigment precipitation reaction such as co-precipitation, or pigment synthesis and crystallization (Patent Documents 6, 7, 8, 12, and 13). When these methods use a reaction involving precipitation or crystallization, a flow path may, with high possibility, be clogged with products or with foams or byproducts generated by the reaction, or the reaction will proceed basically through only diffusion of molecules, so that these methods are not applicable to every reaction. The microchemical process uses a scale-up method of increasing the number of reactors arranged in parallel, but there is a problem that because the manufacturing ability of a reactor is low, large scale up is not practical, and it is difficult for the respective reactors to have equal performance, thus failing to provide uniform products. When the reaction solution is highly viscous or the reaction causes an increase in viscosity, a very high pressure is necessary for passage of the solution through a minute flow path, so there is a problem that a usable pump is limited, and leakage of the solution from an apparatus cannot be solved due to high pressure.
As another growth method, a gaseous phase method using plasma in high vacuum with an apparatus as shown in Patent Document 9 is used in some cases.
In the gaseous phase method, however, the amount of nanoparticles formed at a time is small, an apparatus for an electron beam, plasma, laser or induction heating is necessary for evaporation of raw materials, and there is also a problem in production costs, thus making the gaseous phase method less suitable for mass production. There is also a problem that nanoparticles obtained by the gaseous phase method are fine particles of a pure substance and thus easily aggregated to vary the size of the particles.
Meanwhile, a method of crystallizing a pigment by dissolving the pigment in a fluid in a supercritical or subcritical state and then rapidly cooling the solution is known as a recently reported method (Patent Document 10). This method is carried out at a very high temperature and a high pressure, thus requiring an apparatus capable of realizing an extremely high temperature and pressure. Accordingly, for problems in safety and costs, the method is not practically suitable, particularly not suitable for mass production, and there is also a problem that under such conditions, organic matters such as pigments are easily decomposed.
As a general problem of the apparatus for realizing the methods described above, there is a problem that the cleaning of the apparatus is essential, but the apparatus is poor in cleaning performance because its object material is a pigment. In addition, when contamination with foreign substances and countermeasures against bacteria are also taken into consideration, the apparatus and the production method have not yet reached the practical level for actually obtaining pigment nanoparticles.
Nanoparticles not subjected to pulverization as compared with the nanoparticles produced by pulverization do not have, on the surface of the pigment, a surface roughened by pulverization, and therefore, such pigment nanoparticles have a low speed of aggregation, and as a consequence, the particles are hardly sedimented and are excellent in dispersion stability and also in storage stability. However, there is no production method wherein pigment nanoparticles not having roughened surfaces on the pigment particles can be mass-produced stably and industrially, so there is a demand for establishment of such a production method.
In recent years, the above-mentioned “microchemical process technology” utilizing a micro-reactor or a micro-mixer, that is, a technology wherein a reaction flow path having a minute sectional area is used in a chemical reaction, attracts considerable attention because a chemical reaction can be efficiently conducted. The “microchemical process technology” is a chemical analysis or a method for producing a substance wherein a flow path of several to several hundred μm in width is formed on a solid substrate generally by a microfabrication technology or the like, and a chemical or physicochemical reaction occurring in the microscopic flow path is utilized. However, when the micro-reactor or the micro-mixer is to be realized as an actual manufacturing technology, there are many problems such as necessity for a very high-pressure pump for passing a reaction solution and reactants through the flow path of several μm in width and clogging of the microscopic flow path with products to make the reactor unusable, and in reality, a reactor having a flow path of several hundred μm to several mm in width is required, so it must be said that a technology unworthy to be called a “microchemical process technology” is prevailing under present circumstances.